The proliferation of digital wireless communication systems has led to a need for reliable modulation circuitry. Phase-locked loop (PLL) circuits are attractive in modulation applications due to their combination of controllable modulation and stable and adjustable carrier frequency.
As known in the art, a VCO can be used to effect Gaussian-shaped Continuous Phase Binary Frequency Shift Keying Modulation (GCPBFSK) in which the modulating wave shifts the output frequencies, away from the center frequency, by an amount between predetermined values. The predetermined values, i.e., two different frequencies representing zero and one, are called modulation deviation and are denoted by f0 and f1 respectively. The frequencies, f0 and f1 are typically equal. For example, for a wireless standard having 79 channels in a frequency range of 2402 MHz–2480 MHz with each channel separated by 1 MHz, the center frequencies may be equal to 2402 MHz+nMHz (n=0 to 78). The f0 may range within some negative values (e.g., from −140 KHz to −175 Khz) and f1 may range within a corresponding positive values (e.g. from 140 KHz to 175 KHz), in respect to the center frequency.
In order to keep frequencies f0 and f1 within their specific range, the voltage to the VCO must be controlled very carefully. Typical VCO gain (KVCO) may be on the order of 100 MHz/V–200 MHz/V. Typical VCOs may be operated from 0.6 v to 2.2 v, but actually only the linear range (1.0 v–1.8 v) can be used for this application, otherwise non-linear distortion will result.
For a wide frequency range (2402 MHz–2480 MHz), it is difficult to maintain a linear frequency response. The oscillation frequency of a VCO circuit is inversely proportional to the square root of the product of the inductance and capacitance (LC). It is possible to divide a large frequency range into segments to obtain a piecewise linear response over each segment. This can be achieved by using a set of switch-able capacitors, varactors (whose capacitance is voltage-controllable) and inductors. Often a larger frequency range is divided to account for the tolerance values of integrated capacitors and inductors that may run as high as 30%. Each segment is then covered by a specific capacitor's combination.
The voltage-frequency characteristics of a VCO vary from chip to chip. Therefore to obtain the appropriate modulation characteristics, the voltage to the VCO must be controlled depending on the characteristics of the chip. A typical PLL circuit in accordance with the prior art includes a charge pump, which acts as a current source. The charge pump consists of a set of current sources. The appropriate current source is chosen, based on the VCO voltage-frequency characteristics, to charge the capacitors of the LPF and thus controls the voltage to the VCO.
The difficulty arises in determining the VCO capacitance value (i.e. the capacitors' combination) and charge pump current value to provide the desired modulation deviation for each channel.